WASHINGTON — A federal judge on Thursday backed a finding by government scientists that global warming is threatening the survival of the polar bear.

U.S. District Judge Emmet Sullivan ruled that a May 2008 decision by the U.S. Fish and Wildlife Service to place the bear on the endangered species list as threatened because of melting sea ice was rational given the facts and best available science.

Environmental groups had sued, saying the polar bear needed more protection under the Endangered Species Act. The state of Alaska, under the leadership of then-Gov. Sarah Palin, and hunting groups argued that the listing was unnecessary. They say the bear is protected by other laws and that the scientific case is shaky when it comes to predicting global warming’s toll on the mammal.

In a 116-page opinion, Sullivan dismissed both arguments as “nothing more than competing views about policy and science.”

“According to some plaintiffs, mainstream climate science shows that the polar bear is already irretrievably headed toward extinction throughout its range. According to others, climate science is too uncertain to support any reliable predictions about the future of polar bears,” Sullivan wrote. “This Court is not empowered to choose among these competing views. That is particularly true where, as here, the agency is operating at the frontiers of science.”

The polar bear is unique among species protected under the Endangered Species Act because it is the first to be designated as threatened because of global warming.

That means that while the bear has healthy populations now — some 25,000 can be found across the Arctic region from Alaska to Greenland — scientists had to project what would happen in coming decades. Their conclusion: an estimated 15,000 bears would be lost as rising temperatures caused by the buildup of greenhouse gases in the atmosphere melted the sea ice on which the bears depend.

Another complication was what under the law would be restricted, given that global warming is caused by pollution released far from where the bears live.

When the bear was designated as threatened in 2008, the Bush administration made clear that finding could not be used as a backdoor way to control greenhouse gases. The Obama administration agreed a year later, saying activities outside of the bear’s home — such as emissions from a power plant — could not be controlled using the Endangered Species Act. That exemption is being fought in a separate lawsuit.

The Justice Department, in a statement issued Thursday, said it was pleased that the court agreed with its argument that the decision was based on the science available at the time.

Alaska’s attorney general, John Burns, said, “While the state is disappointed the polar bear remains listed as threatened under the Endangered Species Act, the state believes the court was correct in rejecting the environmental groups’ claim that the polar bear should be listed as endangered.”

Burns added: “However, given the current health of the species the state continues to believe that it is inappropriate to list this species at this time.”

The Center for Biological Diversity, the environmental group that filed the lawsuit, called the decision mixed.

“The good news is the judge strongly rejected the state of Alaska and various industry groups’ arguments that global warming is too uncertain a threat for the bear to be protected at all,” said Brendan Cummings, an attorney for the group. But Cummings said that in the years since the polar bear was listed as threatened, its plight has gotten worse.

Sea ice, which hit an all-time summer low in 2007, has been declining over the past couple of decades and it’s seen a big drop in 2011, said Mark Serreze, director of the National Snow and Ice Data Center. Figures for the past two months show that sea ice levels in the Arctic are currently on a trajectory below the record 2007 year, with weather in the coming month determining if 2011 breaks the 2007 mark, he said.

Serreze said he figures the Arctic will be ice-free in the summertime by 2030. “In terms of where the science has gone, there’s nothing that says we’re not going to lose the summer sea ice. It’s inevitable,” Serreze said.

Polar bears need ice for most of their life functions, but it is most important as a place to forage for seals, their favorite food.

“Even if polar bears could be considered only threatened in 2008, they are clearly endangered today,” Cummings said.

Safari Club International argued against the listing altogether, because it means that the hides of bears hunted in Canada cannot be imported back into U.S., which bans hunting polar bears for sport. It has another lawsuit pending that specifically challenges the importation ban.

“We are disappointed that the judge rejected our arguments that the bear shouldn’t be listed at all,” said the club’s lawyer, Doug Burdin, who said he was still reviewing the ruling and legal options.